The present invention generally relates to thermal transfer protective sheets. More specifically, the present invention relates to thermal transfer protective sheets that include a release layer and a topcoat layer laminated in this order on a base material, as well as prints obtained by using them.
Thermal transfer recording methods using thermal transfer recording media including colored layers such as ink layers formed on a base material are widely employed as methods for recording images on cards such as license cards and credit cards. Thermal transfer recording methods are classified into the fusion thermal transfer recording method and the dye sublimation thermal transfer recording method.
For example, mainstream for thermal transfer recording media used in the fusion thermal transfer recording method have a structure including ink layers mainly including waxes having relatively low melting or softening points formed on a base material consisting of a polyester film or the like so that the ink layers are molten/softened by the heat of a thermal head provided in a printer and transferred to an object to be transferred such as a label, paper, tag or the like, whereby printing is accomplished.
Thermal transfer recording media used in the dye sublimation thermal transfer recording method have ink layers formed by coating sublimatable or thermally transferable dyes to be thermal transferred on a base material consisting of a polyester film or the like so that the sublimatable or thermally transferable dyes in the ink layers are transferred by the heat of a thermal head to an object to be transferred, whereby printing is accomplished.
The images formed by the thermal transfer recording methods as described above suffer from low durability such as weather resistance, mar resistance and chemical resistance. Thus, techniques for improving durability by forming a protective layer on the images formed by the thermal transfer recording methods have been proposed. The protective layer is formed by the steps of superimposing a thermal transfer protective sheet having a base material, a topcoat layer (protective layer) formed on the base material upon an object to be transferred bearing an image, applying heat energy from a thermal head to melt or soften the topcoat layer and ink layers and then cooling/solidifying them to transfer the topcoat layer to the object to be transferred.
In a typical thermal transfer protective sheet, a release layer is formed between the base material and the topcoat layer to improve transferability of the topcoat layer to the object to be transferred and peelability from the base material. See, for example, JPA 2003-127558. Many studies have been made about materials for use in the release layer, among which acrylic resins, especially polymethyl methacrylate (PMMA) resins are known as materials having not only durability after printing but also suitable peelability from the base material.
The fusion thermal transfer recording method is classified into the hot peel method and the cold peel method by the timing at which the heated thermal transfer protective sheet is peeled off from the object to be transferred.
In the cold peel fusion thermal transfer recording method, the topcoat layer and the release layer are sufficiently cooled by leaving a relatively long time interval from heating with a thermal head to peeling. The time interval from heating with a thermal head to peeling depends on the structure of the fusion thermal transfer recording printer, i.e. the distance between the heating element of the thermal head for applying heat energy to the thermal transfer protective sheet and a peeling member functioning to peel off the thermal transfer protective sheet and the object to be transferred from the base material.
Under normal appropriate print energy conditions, peeling is readily accomplished and good prints are obtained so far as the release layer consists of a material that is inherently easy to peel off from the base material such as an acrylic resin because the topcoat layer and the release layer are sufficiently cooled during the time interval from heating with a thermal head to peeling.
When heat accumulates in the thermal head by continuous printing or printing is performed at high applied energy, however, cooling is insufficient in the area from the thermal head to the peeling member because the thermal transfer protective sheet becomes hotter than normal.
If the release layer contains a material having a high glass transition point such as an acrylic resin as major component, the release layer fuses to the base material by residual heat. As a result, peel resistance increases to cause so-called sticking such as wrinkles or breakage of the base material, resulting in troubles such as printer shutdown or printing failure. A similar problem is seen when, for example, a high molecular weight PMMA resin is used in the release layer.
An approach proposed to improve peelability was, for example, to add another layer such as a peeling layer using a silicone resin or the like between the base material and the release layer, but this approach has disadvantages such as an increase in production cost due to complex processes associated with the increased number of layers of the thermal transfer protective sheet.